In the world of relational database management systems (RDBMS), keys play a vital role in ensuring data integrity, eliminating redundancy, and establishing relationships between tables. If you are preparing for database interviews, studying SQL concepts, or working as a database administrator (DBA), itβs essential to understand the difference between Foreign Key, Candidate Key, Composite Key, and Unique Key.
This article breaks down each of these concepts with clear definitions, real-world examples, and use cases.
π What is a Key in DBMS?
A key in DBMS is an attribute (or a set of attributes) that helps uniquely identify records (rows) in a table. Keys enforce rules on data to maintain consistency and prevent anomalies.
1. Candidate Key
- Definition: A candidate key is a minimal set of attributes that can uniquely identify a row in a table.
- Key Point: Every table can have multiple candidate keys, but one of them becomes the primary key.
- Example:
In aStudentstable:student_idemail
Both can uniquely identify a student, hence both are candidate keys.
β SEO Tip: Candidate keys are essential for data uniqueness and are widely asked in SQL interview questions.
2. Composite Key
- Definition: A composite key is a key that consists of two or more attributes together that uniquely identify a record.
- Key Point: A single attribute is not sufficient; the combination makes it unique.
- Example:
In anOrderDetailstable:order_id + product_idβ forms a composite key, since one order can contain multiple products.
β SEO Tip: Composite keys are critical in many-to-many relationships in databases.
3. Unique Key
- Definition: A unique key ensures that the values in a column (or group of columns) remain unique across the table, except it allows one NULL value.
- Difference from Primary Key: Unlike the primary key, which cannot have NULLs, a unique key can.
- Example:
In aUserstable:username(unique key, cannot repeat)email(can also be a unique key)
β SEO Tip: Unique keys are often used to maintain data integrity in columns that are not the primary identifier.
4. Foreign Key
- Definition: A foreign key is a column (or set of columns) in one table that refers to the primary key of another table.
- Purpose: It establishes a relationship between two tables and maintains referential integrity.
- Example:
In anOrderstable:customer_idβ foreign key referencingcustomer_idin theCustomerstable.
β SEO Tip: Foreign keys are crucial in relational database design, ensuring data consistency across tables.
π Key Differences at a Glance
| Key Type | Definition | Allows NULL | Example |
|---|---|---|---|
| Candidate Key | A minimal set of attributes that uniquely identify a row | No | student_id, email |
| Composite Key | A combination of two or more attributes used as a key | No | order_id + product_id |
| Unique Key | Ensures all values are unique in a column, allows one NULL | Yes (one NULL) | username, email |
| Foreign Key | Refers to the primary key of another table | Yes | customer_id in Orders table |
π Conclusion
Understanding the difference between Foreign Key, Candidate Key, Composite Key, and Unique Key is fundamental for mastering SQL database design. These keys are not only essential for maintaining data accuracy and integrity but also form the backbone of RDBMS interview questions.
If youβre preparing for SQL interviews or working on database projects, make sure you can clearly explain:
- When to use a Candidate Key vs Primary Key
- How Composite Keys handle complex relationships
- Why Unique Keys allow NULLs
- How Foreign Keys enforce referential integrity
